Thermal ink-jet printers operate by employing a plurality of resistor elements to expel droplets of ink through an associated plurality of nozzles. In particular, each resistor element, which is typically a pad of a resistive material measuring about 50 .mu.m.times.50 .mu.m, is located in a chamber filled with ink supplied from an ink reservoir. A nozzle plate, comprising a plurality of nozzles, or openings, with each nozzle associated with one of the resistor elements, defines the top of the chamber. Upon energizing of a particular resistor element, a droplet of ink is expelled through the nozzle associated with that element toward the print medium, whether paper, transparent film, or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements, thereby enabling the formation of alphanumeric and other characters on the print medium.
The tight tolerances of the nozzles (typically 40 to 50 .mu.m diameter) require that the ink not clog the nozzles. Nozzle clogging is caused by loss of vehicle by evaporation. Such evaporation results in either an increase in viscosity such that the nozzle can no longer fire or the dye and other additives come out of solution due to increases in their effective concentration. Consequently, it is desirable to use co-solvents which have vapor pressures less than that of water and viscosities less than 35 centipoise (cp) in conjunction with alcohol as the penetrant, as described hereinbelow.
Further, repeated firings of the resistor elements, which must withstand many millions of firings over the life of the ink cartridge to be commercially practical, can result in fouling of the resistor element. This is unique to thermal ink-jet printers and is known as kogation.
The ink composition must be capable of interacting with the print medium, especially paper, to penetrate the medium without undue spreading and a subsequent decrease in print quality. Finally, the printed characters must dry rapidly in order to prevent smearing.
Inks are known which possess one or more of the foregoing properties. However, few ink compositions are known that possess all the foregoing properties, since an improvement in one property often results in the degradation of another property. Thus, many inks used commercially represent a compromise in an attempt to achieve an ink evidencing at least an adequate response in each of the foregoing considerations. Accordingly, investigations continue into developing ink formulations which have improved properties and which do not elevate one property at the expense of the others.